Sea-Bird Electronics SBE 911 and SBE 917 series CTD profilers

The SBE 911 and SBE 917 series of conductivity-temperature-depth (CTD) units are used to collect hydrographic profiles, including temperature, conductivity and pressure as standard. Each profiler consists of an underwater unit and deck unit or SEARAM. Auxiliary sensors, such as fluorometers, dissolved oxygen sensors and transmissometers, and carousel water samplers are commonly added to the underwater unit.

Underwater unit

The CTD underwater unit (SBE 9 or SBE 9 plus) comprises a protective cage (usually with a carousel water sampler), including a main pressure housing containing power supplies, acquisition electronics, telemetry circuitry, and a suite of modular sensors. The original SBE 9 incorporated Sea-Bird's standard modular SBE 3 temperature sensor and SBE 4 conductivity sensor, and a Paroscientific Digiquartz pressure sensor. The conductivity cell was connected to a pump-fed plastic tubing circuit that could include auxiliary sensors. Each SBE 9 unit was custom built to individual specification. The SBE 9 was replaced in 1997 by an off-the-shelf version, termed the SBE 9 plus, that incorporated the SBE 3 plus (or SBE 3P) temperature sensor, SBE 4C conductivity sensor and a Paroscientific Digiquartz pressure sensor. Sensors could be connected to a pump-fed plastic tubing circuit or stand-alone.

Temperature, conductivity and pressure sensors

The conductivity, temperature, and pressure sensors supplied with Sea-Bird CTD systems have outputs in the form of variable frequencies, which are measured using high-speed parallel counters. The resulting count totals are converted to numeric representations of the original frequencies, which bear a direct relationship to temperature, conductivity or pressure. Sampling frequencies for these sensors are typically set at 24 Hz.

The temperature sensing element is a glass-coated thermistor bead, pressure-protected inside a stainless steel tube, while the conductivity sensing element is a cylindrical, flow-through, borosilicate glass cell with three internal platinum electrodes. Thermistor resistance or conductivity cell resistance, respectively, is the controlling element in an optimized Wien Bridge oscillator circuit, which produces a frequency output that can be converted to a temperature or conductivity reading. These sensors are available with depth ratings of 6800 m (aluminium housing) or 10500 m (titanium housing). The Paroscientific Digiquartz pressure sensor comprises a quartz crystal resonator that responds to pressure-induced stress, and temperature is measured for thermal compensation of the calculated pressure.

Additional sensors

Optional sensors for dissolved oxygen, pH, light transmission, fluorescence and others do not require the very high levels of resolution needed in the primary CTD channels, nor do these sensors generally offer variable frequency outputs. Accordingly, signals from the auxiliary sensors are acquired using a conventional voltage-input multiplexed A/D converter (optional). Some Sea-Bird CTDs use a strain gauge pressure sensor (Senso-Metrics) in which case their pressure output data is in the same form as that from the auxiliary sensors as described above.

Deck unit or SEARAM

Each underwater unit is connected to a power supply and data logging system: the SBE 11 (or SBE 11 plus) deck unit allows real-time interfacing between the deck and the underwater unit via a conductive wire, while the submersible SBE 17 (or SBE 17 plus) SEARAM plugs directly into the underwater unit and data are downloaded on recovery of the CTD. The combination of SBE 9 and SBE 17 or SBE 11 are termed SBE 917 or SBE 911, respectively, while the combinations of SBE 9 plus and SBE 17 plus or SBE 11 plus are termed SBE 917 plus or SBE 911 plus.

Paroscientific Absolute Pressure Transducers Series 3000 and 4000

Paroscientific Series 3000 and 4000 pressure transducers use a Digiquartz pressure sensor to provide high accuracy and precision data. The sensor comprises a quartz crystal resonator that responds to pressure-induced stress, and temperature is measured for thermal compensation of the calculated pressure.

The 3000 series of transducers includes one model, the 31K-101, whereas the 4000 series includes several models, listed in the table below. All transducers exhibit repeatability of better than ±0.01% full pressure scale, hysteresis of better than ±0.02% full scale and acceleration sensitivity of ±0.008% full scale /g (three axis average). Pressure resolution is better than 0.0001% and accuracy is typically 0.01% over a broad range of temperatures.

Differences between the models lie in their pressure and operating temperature ranges, as detailed below:

BODC Processing

Conductivity, temperature, salinity and conductivity from the primary sensors were received in a structured matlab format and converted into BODC internal format. The auxiliary sensor data were not processed by the originators and will be loaded separately. Potential temperature of the water body was derived by computation using UNESCO 1983 algorithm, σθ was derived by computation from salinity and potential temperature also using UNESCO 1983 algorithm. In addition to the variables loaded, the matlab file also contained; conductivity, latitude, longitude and potential temperature. The following table shows how the variables within the matlab file were mapped to appropriate BODC parameter codes:

Originator's Parameter Name

Units

Description

BODC Parameter Code

Units

Comments

temp

°C

Temperature from primary sensor

TEMPS901

°C

-

cond

mS cm-1

Conductivity

CNDCST01

S/m

Conversion needed (/10)

sal1

-

Practical salinity

PSALCC01

Dimensionless

Calibrated against CTD bottle salinity samples

pres

dbar

Pressure exerted by the water column

PRESPR01

dbar

-

-

-

Potential temperature

POTMCV01

°C

Generated by BODC using the Fofonoff and Millard (1983) algorithm

-

-

σθ of the water column

SIGTPR01

kg m-3

Generated by BODC using the Fofonoff and Millard (1983) algorithm

The reformatted data were visualised using the in-house EDSERPLO software. The data were screened and quality control flags were applied to data as necessary.

References

Originator's Data Processing

Sampling Strategy

In total, 10 CTD stations were carried out, this included six CTD casts (CTD001 to CTD006) and four tow-yo casts (CTD007 to CTD010). The first two tow-yo casts (CTD007 to CTD008) were full-depth tow-yos with three up-and-down profiles each. The last two tow-yo casts (CTD009 to CTD010) were in shallow water with 14 and 24 up-and-down profiles respectively. Four casts (CTD003 to CTD006) were carried out in the location of the moorings deployments. In total, 94 profiles (including upcasts) were obtained with no major operational issues encountered.

Data processing

The files were produced by Seawave (v 7.18) and initial data processing was performed using SBE Data Processing, Version 7.19. Firstly, the raw data were converted into physical units using 'Data Conversion'. A temporal offset of 5 was then applied to align the oxygen sensor readings using 'Align CTD'. Offsets were not applied for primary and secondary temperature and salinity, as the CTD deck unit automatically applied the conductivity lag to the conductivity sensors. Corrections for the thermal mass of the cell were made using 'Cell Thermal Mass' and the output from the 'Align CTD', in order to minimise salinity spiking in steep vertical gradients due to temperature/conductivity mismatch. 'Filter' applied a low pass filter to the pressure channel, followed by 'Loop Edit' which applied flags to pressure reversals, where the package has slowed down or even stopped. After which, the surface soak was removed followed by 'Derive' which calculated primary and secondary salinity and oxygen concentrations.

After the Sea-Bird processing, further processing was undertaken using the National Oceanography Centre's Mstar software package using the MEXEC processing suite of programs. Firstly, sample (SAM) files were created to store all information about rosette bottle samples. The CTD data were then processed, with the data firstly averaged to 1 Hz, followed by the calculation of practical salinity and potential temperature. For the CTD casts, the downcast data were extracted to create a 2 db resolution file. Positions were loaded to from the navigation file. For the tow-yo casts each upward and downward profile was isolated and the data averaged and interpolated into a 2 db bins and then manually appended into one file.

Further details of the originator's processing can be found in the cruise report.

Field Calibrations

Salinity

Ninty eight from a total of 109 salinity samples from the CTD Niskin bottle samples were used to calibrate the CTD salinity data. A comparison of the primary and secondary sensors showed there to be drift in the second sensor seemingly linear in depth. Therefore, only data from the primary sensor were used for the calibration. In addition, the difference between CTD salinity and the bottle salinity was greater than usual with some discussion about the possible reasons given in the cruise report. The primary salinity data were calibrated but this calibration equation is unknown. The difference between the uncalibrated and calibrated salinity was determined to be +0.0013.

DIMES is a US/UK field program aimed at measuring diapycnal and isopycnal mixing in the Southern Ocean, along the tilting isopycnals of the Antarctic Circumpolar Current.

The Meridional Overturning Circulation (MOC) of the ocean is a critical regulator of the Earth's climate processes. Climate models are highly sensitive to the representation of mixing processes in the southern limb of the MOC, within the Southern Ocean, although the lack of extensive in situ observations of Southern Ocean mixing processes has made evaluation of mixing somewhat difficult. Theories and models of the Southern Ocean circulation have been built on the premise of adiabatic flow in the ocean interior, with diabatic processes confined to the upper-ocean mixed layer. Interior diapycnal mixing has often been assumed to be small, but a few recent studies have suggested that diapycnal mixing might be large in some locations, particularly over rough bathymetry. Depending on its extent, this interior diapycnal mixing could significantly affect the overall energetics and property balances for the Southern Ocean and in turn for the global ocean. The goals of DIMES are to obtain measurements that will help us quantify both along-isopycnal eddy-driven mixing and cross-isopycnal interior mixing.